1. Field of the Invention
The present invention relates to an apparatus and a method for measuring and controlling a trajectory (track) of a droplet target to be used for generating plasma radiating EUV (extreme ultraviolet) light in a chamber apparatus of an LPP (laser produced plasma) type EUV light source apparatus for generating EUV light to be used for exposure of semiconductor wafers or the like.
2. Description of a Related Art
In recent years, as semiconductor processes become finer, photolithography has been making rapid progress toward finer fabrication. In the next generation, microfabrication at 60 nm to 45 nm, further, microfabrication at 32 nm and beyond will be required. Accordingly, in order to fulfill the requirement for microfabrication at 32 nm and beyond, for example, exposure equipment is expected to be developed by combining an EUV light source for generating EUV light having a wavelength of about 13 nm and reduced projection reflective optics.
As the EUV light source, there are three kinds of light sources, which include an LPP (laser produced plasma) light source using plasma generated by irradiating a target with a laser beam, a DPP (discharge produced plasma) light source using plasma generated by discharge, and an SR (synchrotron radiation) light source using orbital radiation. Among them, the LPP light source has advantages that extremely high intensity close to black body radiation can be obtained because plasma density can be considerably made larger, and that the light of only the particular waveband can be radiated by selecting the target material. Further, the LPP light source has advantages that an extremely large collection solid angle of 2π to 4π steradian can be ensured because it is a point light source having substantially isotropic angle distribution and there is no structure such as electrodes surrounding the plasma light source and so on. Therefore, the LPP light source is considered to be predominant as a light source for EUV lithography, which requires power of more than several tens of watts.
Here, a principle of generating EUV light in the LPP light source will be explained. A target material supplied into a chamber is irradiated with a driver laser beam, and thereby, the target material is excited and turned into plasma. From the plasma, various wavelength components including EUV light are radiated. Then, EUV light is reflected and collected by using an EUV collector mirror for highly reflecting a specific wavelength component (for example, a component having a wavelength of 13.5 nm), and outputted to a device using EUV light (for example, exposure unit). For the purpose, on a reflection surface of the EUV collector mirror, for example, a multilayer coating (Mo/Si multilayer coating) in which molybdenum (Mo) thin coatings and silicon (Si) thin coatings are alternately stacked is formed.
In the LPP light source, as the target material to be used for generating plasma radiating EUV light, liquid tin is considered to be a predominant material. Accordingly, in the LPP light source, a target delivery mechanism is provided for injecting tin melted at a high temperature from a target injection nozzle and supplying it in a droplet state to a predetermined plasma generation position. Here, the predetermined plasma generation position is a position on which a pulse laser beam is focused by using a laser beam focusing optics, and, when the target material passing through the position is irradiated with the pulse laser beam, plasma is generated.